Why did I choose to go to Chichester to see this play? I don’t know, and neither does anybody else. The facts are clear enough. I had seen it was coming, and thought it would be interesting. But I didn’t book tickets until the last minute. I didn’t realise how popular it would be. How could I? So by the time I came to book nearly every seat was taken and I had very little choice of which seat to take – and there were no nights where two seats were left next to each other. Is that why I went alone? Or did my wife’s reluctance to go and see a play about a couple of physicists with a total cast of 3 and no prospect of any singing or dancing have something to do with it. Did that hold me back from getting my credit card out. Did I only commit when I had a valid excuse for why I was going alone? I literally don’t know the answers to these questions, even though it all happened in my head in the last month. Our brains and how they work are a mystery to ourselves. Michael Frayn could probably get a play out of this.

He’s already done a pretty good job of examining why Werner Heisenberg visited Neils Bohr in Copenhagen in 1941. Spoiler alert – we never find out why. This of course leaves the possibility of a sequel on the table. “Copenhagen II – Heisenberg’s back , and more uncertain than ever.” This is a remarkable play. It covers human relationships, memory, quantum mechanics, the way science is done and history. If plays could be written like equations, this is one with multiple solutions all of which have been solved with accuracy and precision.

The play does however have one big question. Could Nazi Germany have created an atomic bomb? Not many people know enough about the practicalities of creating an entirely new form of weapon from cutting edge scientific research to have an opinion. But Werner Heisenberg was not just familiar with the science – he had created some of it. And he knew most of the other players in it. And he was well connected in Germany and was the head of one of the two programmes charged with developing a bomb for the Fuhrer. If anyone knew, it was him.

This makes his meeting with Neils Bohr so fascinating. What was he trying to do? Was he attempting to get information on the American project? Was he trying to get help? Both men survived the war and were later able to comment on what they had discussed. But while they were both undoubtedly men with great minds, they don’t seem to have been any better at remembering what happened than the rest of us. Their accounts don’t agree on the date, or the location of their discussions. I speak as someone who has taken to going upstairs as quickly as possible. If I don’t I will forget the reason I am going up them by the time I reach the top. It is sort of comforting to know that the best of us have similar problems. But they agree that Heisenberg strongly hinted at the existence of the German programme – a revelation that would have been treason during wartime – and enquired about the progress on the allied side. He also proposed a tacit understanding between the physicists on both sides to subvert the progress and prevent the weapon coming into the hands of either side. These scientists all knew one another, so that was a plausible suggestion.

Even this much detail poses more questions than it answers. Was Heisenberg really being idealistic? Or did he know that the German project was hopeless and so was trying to prevent his country ending up at a disadvantage. Would he have subverted the development if he was able to? In fact was he able to and actually did so? All these possibilities are examined in the play. One further possibility is that Heisenberg, brilliant though he was, had made a very simple and human error. It is possible to calculate just how much material you need to make an atomic bomb. It is not a trivial calculation, and in the days before pocket calculators let alone spreadsheets, it would have taken some time to carry out. It was nonetheless something that any reasonably good scientist could have done and was well within Heisenberg’s capabilities. I work in a lab myself, and I’ll confess that sometimes making a rough guess rather than running the actual numbers is a temptation I’ve succumbed to from time to time.

Heisenberg’s rough guess was that around a tonne would be needed. This was a practical impossibility for Germany and a pretty tall order for the United States. The correct figure was 50Kg – challenging but doable. If the Germans had had a more realistic target could they have made the necessary progress in the time available? Would 1945 have seen London and Moscow reduced to ashes? I don’t know. Nobody will ever know.

But don’t let me give you the impression that this play is just an historical whodunnit. That’s just one aspect of it – fascinating as it is. There’s a lot more going on and if you get a chance to see it I thoroughly recommend it. Even though I’m writing this while it is still on in Chichester you are too late to get a ticket as the season is sold out. But if the same cast,Charles Edwards, Patricia Hodge and Paul Jesson ever do it again make sure you book early.